There are significantly improved required peak rates of a Long Term Evolution-Advanced (LTE-A) system up to 1 Gbps in the downlink and 500 Mbps in the uplink as compared with an LTE system. Also the LTE-A system is required to be well compatible with the LTE system. Carrier Aggregation (CA) has been introduced to the LTE-A system to thereby accommodate the improved peak rates, compatibility with the LTE system and full use of spectrum resources as needed.
With carrier aggregation, a User Equipment (UE) can operate concurrently on a plurality of cells, where these cells can be consecutive or inconsecutive in frequency, and bandwidths of the respective cells may be the same or different. There is a limited bandwidth up to 20 MHz of each cell for compatibility with the LTE system, and the number of cells that can be aggregated for one user equipment is typically up to 5 at present.
In the carrier aggregation-enabled system, all the cells configured by an evolved Node B (eNB) for the user equipment are referred to as serving cells, but all the functions of the different serving cells may not be the same, so the serving cells are further categorized in the LTE-A system as follows:
A Primary Cell (PCell), where only one of the plurality of cells aggregated for the user equipment is defined as a PCell selected by the evolved Node B and configured to the user equipment via Radio Resource Control (RRC) signaling. A Physical Uplink Control Channel (PUCCH) is configured only on the PCell; and
A Secondary Cell (SCell), where all the other cells than the PCell aggregated for the user equipment are SCells.
Power Headroom (PH) information for configured and activated cells needs to be reported in a Power Headroom Report (PHR) defined in the LTE-A Release 10 (R10), that is, the PH information for all the configured and activated cells is packaged and reported once the PHR is triggered.
Only aggregation of carriers controlled by the same eNB for the UE is supported in the LTE-A R10 and earlier releases, and also only aggregation of carriers controlled by the same eNB for the UE is supported in the existing PHR mechanism. If carriers controlled by different eNBs can be aggregated for the UE in later releases than the LTE-A R10, then the existing PHR mechanism can not be applicable any longer.
In summary, there has been absent so far a solution to reporting power headroom in the scenario where carriers controlled by different eNBs are aggregated for the UE.